FIG. 2 shows a prior art narrow band disturbing wave detecting device, in which reference numeral 11 is a disturbing wave detecting circuit and 12 is a variable notch filter. Disturbing wave in a received signal in a desired frequency band is detected by the disturbing wave detecting circuit 11. The disturbing wave in the received signal is removed to output only desired wave as an output signal by making the notch filter frequency of the variable notch filter 12 in accordance with a disturbing frequency corresponding to a detected signal thus obtained.
However such a prior art method has a problem that the construction of the device is complicated and expensive, because a special disturbing wave detecting circuit is required for detecting disturbing wave.